1. Field of Invention
The present invention relates to an electrostatic printing device for forming an image using toner particles in printers, facsimiles, copying machines and so on, and parts therefor.
2. Description of the Prior Art
Printing devices, by which electric signals output from computers, word processors, facsimiles, or the like are formed as visible images on a recording medium such as paper or the like, include an electrostatic printing device 1 shown in FIG. 5, in which an electrode unit is arranged between a particle carrier and a back electrode.
The electrostatic printing device 1 generates an electric potential difference between the particle carrier 20 and the back electrode 30 to create an electric field, by which toner particles are conveyed toward the back electrode 30 from the particle carrier 20, and the electrode unit 10 arranged between the particle carrier 20 and the back electrode 30 controls conveyance of toner toward the back electrode 30 from the particle carrier 20 to enable forming of a desired image on a recording medium 50, such as paper or the like, or an intermediate recording medium, such as a transfer belt or the like, disposed between the particle carrier 20 and the back electrode 30.
The above-mentioned electrode unit 10 comprises apertures 11 and control electrodes 12 surrounding the apertures 11 at least partially (see FIG. 6), and voltage applied on the control electrodes 12 has an influence on an electric field, by which toner particles are conveyed toward the back electrode 30, so that toner particles conveyed toward the back electrode 30 from the particle carrier 20 determine positions, sizes and the like of dots formed on the recording medium 50.
The electrode unit 10 is formed from a base material of a resin film or resin sheet composed of, for example, a resin material such as polyimide or the like and having a thickness of around 25 to 200 xcexcm, the base material being formed with a plurality of apertures 11 aligned in a predetermined direction, and the apertures 11 being formed to be at least partially surrounded by, for example, mutually intersecting control electrodes or the control electrodes 12 formed in a ring-shaped fashion.
When being assembled into the printing device 1, the electrode unit 10 is disposed between the particle carrier 20 and the back electrode 30 such that rows of the apertures (L1 to L4) of the unit 10 are made in parallel to an axis of the particle carrier 20, which is formed as a column or cylinder-shaped rotating body.
In the example shown in FIG. 5, the electrode unit 10 is formed with four rows of apertures (L1 to L4) disposed in parallel. In the case where the plural rows of apertures (L1 to L4) are provided in the electrode unit 10, a distance between the surface of the particle carrier 20 and the apertures 11 formed on the electrode unit 10 varies depending upon, for example, to which of the rows of apertures (L1 to L4) the apertures 11 belong, when the electrode unit 10 is positioned in a planar manner.
In the specification of the present application, assuming that Lk denotes a distance between the respective apertures 11 of the electrode unit 10 and the surface of the particle carrier 20, Lk is large between the apertures 11 belonging to the right and left rows (L1, L4) in FIG. 5 and the surface of the particle carrier 20 and small between the apertures 11 belonging to the central rows (L2, L3) and the surface of the particle carrier 20. Also, since even with an electrode unit 10 comprising one or two rows of apertures, it is difficult to arrange the row or rows of apertures in completely parallel to the axis of the particle carrier, Lk is in some cases varied in the apertures 11 belonging to the same row. Therefore, as Lk is varied, the control electrodes 12 have different influences on an electric field formed between the particle carrier 20 and the back electrode 30 even in the case where the same voltage is applied to the control electrodes 12 surrounding the apertures 11 in the respective rows (L1 to L4) of apertures, so that dots formed on the recording medium 50, such as paper or the like, vary in size and density depending upon which of the control electrodes 12 surrounding the apertures 11 has controlled the forming of the dots.
By way of example, in the case where dots are formed on the recording medium 50 assuming that all the control electrodes 12 surrounding the apertures 11 (L1 to L4) formed on the electrode unit 10 in the printing device 1 shown in FIG. 5 are the same in electric potential, when the apertures 11 belonging to the rows L2, L3 and having a relatively small distance Lk between them and the surface of the particle carrier 20 form relatively deep and large dots, and the apertures 11 belonging to the rows L1, L4 and having a relatively small distance Lk between them and the surface of the particle carrier 20 form light and small dots, quantity of toner particles adhered to a printed surface finished and sizes of dots formed differ depending upon, through which of the apertures 11 adherence of toner particles and formation of dots are made, when dots are consecutively shown in, for example, FIG. 6, so that there are produced areas being uneven in density and not printed, such unevenness and non-printed areas being visually recognized as lines.
Such phenomenon is called xe2x80x9cwhite line noisexe2x80x9d, which causes degradation in printing quality, and removal of which is contemplated. In order to prevent generation of such xe2x80x9cwhite line noisexe2x80x9d, distances Lk between the surface of the particle carrier 20 and the apertures 11 formed on the electrode unit 10 are made constant to eliminate variation xcex94Lk in the distances, thereby solving the problem of xe2x80x9cwhite line noisexe2x80x9d. Therefore, there has been proposed a printing device (see FIG. 7) constructed such that distances Lk between the apertures 11 of the electrode unit 10 and the surface of the particle carrier 20 are made uniform in all the rows (L1 to L4) by bending that area of the electrode unit 10, in which the apertures 11 are formed, so that all the apertures 11 are adjusted to be disposed on a circle concentric with an outer periphery of the particle carrier 20.
As described above, with the printing device 1, in which the area of the electrode unit 10 formed with the apertures 11 is bent in compliance with the surface configuration of the particle carrier 20, all the distances Lk between the surface of the particle carrier 20 and the respective rows (L1 to L4) of the apertures of the electrode unit 10 are uniform, and therefore it is possible to prevent that degradation in printing quality, which is attributed to unevenness in such distances.
When the electrode unit 10 formed from a base material of a resin film or resin sheet is bent in a certain direction, the cross section of the bent portion itself undergoes deformation in its inner surface with the result that such deformation causes the electrode unit 10 to change in shape.
For example, when an Xxe2x80x94X axis of an electrode unit 10 put in a state of being disposed in a planar position shown in FIG. 8 is bent into a shape shown by a broken line Xxe2x80x2xe2x80x94Xxe2x80x2 as shown in FIG. 9, the electrode unit 10 generates warp xcfx81 at both ends in the direction along a Zxe2x80x94Z axis perpendicular to the Xxe2x80x94X axis thus bent (see FIG. 9).
FIG. 10 shows a state of a minute space containing an origin O and cut from the bent portion of the electrode unit 10 put in the state shown in FIG. 9. When the electrode unit 10 is bent in a widthwise direction in a square column above the origin O (above the broken line) in FIG. 10, normal stress acts to compress the electrode unit 10 in the direction along the Xxe2x80x94X axis, thereby generating longitudinal strain xcex5x and lateral strain xcex5x(=xe2x88x92xcexdxcex5x) in the direction along the Zxe2x80x94Z axis since the electrode unit 10 compressed by the normal stress tends to expand in a lateral direction.
Also, stress and strain in a state opposite to the above are generated in a square column below the origin O (below the broken line) in FIG. 10. Therefore, when the electrode unit 10 is bent in the direction along the Xxe2x80x94X axis, stress is generated in the direction along the Zxe2x80x94Z axis to tend to bend the electrode unit 10. This stress is liable to be released at ends of the substrate, so that xe2x80x9cwarpxe2x80x9d which affects the print quality is caused in both end portions of the substrate in the direction along the Zxe2x80x94Z axis (see FIG. 11).
Thus, when the electrode unit 10 is bent in the direction perpendicular to the rows of apertures, xe2x80x9cwarpxe2x80x9d is produced in both longitudinal end portions of the rows of apertures, so that degradation in printing quality is caused in the case where apertures 11 are formed up to end portions of the electrode unit 10 and all the apertures 11 are used for formation of an image. That is, the apertures 11 formed in both end portions of the electrode unit 10 are displaced in directions away from the surface of the particle carrier 20 to make distances Lk large to generate dispersion between the apertures 11 in the same rows. Therefore, in the case where such displacement is generated to a degree having an influence on printing quality, that is, beyond an error in an allowable range, dots formed are different in size and density between the respective apertures 11 to cause degradation in printing quality.
However, there have not been conventionally proposed any method and device for dissolving that degradation in image quality, which is caused by xe2x80x9cwarpxe2x80x9d generated in both longitudinal end portions of an electrode unit 10.
Accordingly, the present invention has been made with a view to overcoming the above disadvantages of the prior art and has its object to provide an electrode unit capable of forming an image of excellent quality by eliminating variations in distances between apertures formed on an electrode unit and the surface of a particle carrier as far as possible even if xe2x80x9cwarpxe2x80x9d is generated at both end portions of rows of apertures in a longitudinal direction of rows of apertures when the electrode unit is bent in the direction perpendicular to the rows of apertures.
In order to attain the above object, an electrostatic printing device 1 and an electrode unit 10 used for the electrostatic printing device 1 according to the present invention are provided, the electrostatic printing device comprising a column-shaped or cylinder-shaped particle carrier 20 carrying toner particles on the surface thereof, a back electrode 30 disposed opposite the particle carrier 20 and an electrode unit 10 arranged between the particle carrier 20 and the back electrode 30 and formed with a plurality of apertures 11, which are at least partially surrounded by control electrodes 12, and wherein electric voltage producing an electric potential difference between the particle carrier 20 and the back electrode 30 and applied to the control electrodes 12 of the electrode unit 10 controls conveyance of toner particles toward the back electrode 30 from the particle carrier 20 to form a desired image on a recording medium 50 disposed between the particle carrier 20 and the back electrode 30, and the electrode unit 10 comprises a substrate formed from a resin film or resin sheet, the apertures 11 being arranged in a predetermined direction of the substrate to provide rows of apertures (L1, L2), the rows of apertures (L1, L2) on the electrode unit 10 being arranged in the direction parallel to an axis of the particle carrier 20, the electrode unit 10 being arranged to be curved in the direction perpendicular to the longitudinal direction of the rows of apertures (L1, L2), and the respective rows of apertures (L1, L2) being positioned equidistantly from an outer periphery of the particle carrier 20, and wherein when the substrate is bent in the direction perpendicular to the rows of apertures (L1, L2), the substrate curves in the longitudinal direction of the rows of apertures (L1, L2) to create an image forming area 16 defined by that portion on the substrate, in which displacements (variations xcex94Lk) produced in directions away from the surface of the particle carrier 20 are in a predetermined range of allowable error, and image unforming areas 15, 15 being not used for formation of an image and defined by those areas, which extend from both ends of the image forming area to both ends of the substrate.
In addition, the image unforming areas 15, 15 may comprise areas, in which the apertures 11 are not formed, and the apertures 11 may be formed. However, the apertures 11 formed in the image unforming areas 15, 15 are not used for formation of an image.
The image unforming areas 15, 15, respectively, are provided to extend over a length fifty times or more, more preferably, two hundred times or more a thickness of the substrate in the longitudinal direction of the rows of apertures.